Ellipsometer is an optical instrument using a monochromic polarized light to estimate characteristics of a sample. One common application of ellipsometer is to estimate the thickness of a thin film. The conventional ellipsometer, however, include a rotation mechanism for mechanically rotating a polarizer or a wavelength retarder to change the polarization direction of the incident light during estimation, such that a plurality sets of ellipsometric parameters corresponding to different polarization states can be sequentially obtained. Accordingly, the thickness of the thin film can be estimated based on the plurality sets of ellipsometric parameters. The conventional approach, however, takes a long time to obtain the required ellipsometric parameters, and thus is difficult to in-situ measure the thickness of the thin film during a manufacturing operation.